Embodiments of the invention relate generally to magnetic resonance (MR) imaging, and more particularly, to MR based tracking of a tissue point using a double pencil beam RF pulse.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR (nuclear magnetic resonance) signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In addition to being used for the purpose of standard medical imaging procedures, MR imaging has increasingly been integrated into surgical procedures. That is, MR imaging is now used as a guidance system in certain surgical or therapy procedures. One such procedure is focused ultrasound therapy and/or surgery, in which ultrasound energy is delivered to localized regions of tissue from externally-located (non-invasive) or internally-located (minimally invasive) transducers. The amount of ultrasound energy delivered to tissue dictates the nature of the biologic effect produced at that location and can, for example, induce thermal coagulation of tissue, vascular occlusion or hemorrhage, permeation of cells, and tissue-homogenization.
When performing focused ultrasound procedures, it is desirable to provide precise control and steering of the acoustic field by making use of MR images. MR imaging enables precise targeting of structures for treatment planning, on-line temperature mapping and imaging for monitoring and control of therapy, and results in excellent visualization of the biological response to treatment. That is, an MRI system may be used to plan a focused ultrasound procedure by performing an initial scan to locate a target tissue region and/or to plan a trajectory between an entry point and the tissue region in preparation for a procedure. Once the target tissue region has been identified, MRI may be used during the procedure, for example, to image the tissue region and/or to guide the trajectory of an external ultrasound beam to a target tissue region being treated. In addition, an MRI system may be used to monitor the temperature of the tissue region during the procedure, for example, to ensure that only the target tissue region is destroyed during an ablation procedure without damaging surrounding healthy tissue.
The physiological motion of tissue in vivo, whether due to the respiratory cycle or cardiac cycle, can make it difficult to track a target point of tissue that is being treated by the focused ultrasound when that target point is constantly in motion. Real-time tracking during focused ultrasound ablation is important in avoiding the heating of an unwanted area and in delivering focused ultrasound to target location so as to reach a target temperature thereat. While various tissue point tracking techniques are currently in use, such techniques typically use preliminary imaging procedures (i.e., a learning phase) and/or may experience data latency issues that affect the timeliness of the ultrasound application.
It would therefore be desirable to have a system and method of target point tracking in tissue that allows for direct measurement/tracking of the location of the target point without a learning phase. It is further desired that the system and method minimize data latency in the target point tracking so as to improve the accuracy and timeliness of location data on the target point.